1. Field of Invention
The present invention relates to a method for producing zinc oxide, and more particularly to a method for producing high-purity nanometer zinc oxide.
2. Description of Related Arts
At present, the zinc hydrometallurgy mainly adopts the production process of roasting, leaching and electrolysis. In the acid leaching residue which is discharged by said production process, the mass percent content of zinc is in general 6˜9%, and sometime as high as 20%. The phase analysis and X-ray difpercent analysis made for said acid leaching residue have shown that, the zinc in said leaching residue mainly exists in a form of ZnFe2O4 and zinc silicate. Meanwhile, the mass percent of sulfur is 6-12%, of which, the mass percent of sulfate radical is 15-30%. The content of calcium and magnesium in the mine vary, with the ions such as Cu2+, Pb2+, Cd2+, Ni2+, Hg2+, As3+, and the mass percents are as follows: copper 1%, lead 0.8%, iron 4%, silicon 3%, arsenic 0.3%, silver 0.003%, calcium 12%, and magnesium 3%. In order to recycle the zinc, the current treatment methods include pyrometallurgy and hydrometallurgy. Said pyrometallurgy contains kiln volatilization method (namely Waelz process) and fuming furnace volatilization method, such as the method of Chinese patent application with publication number CN101886180. Said hydrometallurgy contains hot acid leaching and high temperature & pressure leaching. The pyrometallurgy need a long technological process, large amount of equipment maintenance, high investment, poor working conditions, consume a large amount of coal or metallurgical coking coal, and the efficiency is low and environmental pollution is great. Therefore, it usually adopts hot acid leaching or high pressure leaching. These methods still have the drawbacks as follows: {circle around (1)} consuming a large amount t of acid, {circle around (2)} high pressure of iron removal, consuming more reagents, {circle around (3)} equipments have serious corrosion owing to high temperature and high pressure. The equipments are complex, with high investment; {circle around (4)} high operating costs, low economic efficiency. {circle around (5)} the discharge of these present methods is acid residue, so they bring new pollution, it has no choice but to bury, not only pollute the environment, but also a waste of resources.
The most ideal processing method for the acid leaching residue is the selective leaching of zinc, it make the zinc entering into the final leached solution, and recycle zinc valuably.
High-purity zinc oxide usually refers to the zinc oxide product with the mass percent ≧99.7%. The high-purity zinc oxide is an indispensable raw material for the modern high technologies, with wide applications. It is mainly used in glass, feed, ceramics, dyes, paint, paper-making, rubber, pesticides, oil refining, galvanization, special steel, alloy, defence-related science and technology, etc. The glass, paper-making, or rubber, oil refining enterprises have high demands for zinc oxide and very high purity requirement.
Nanometer zinc oxide (ZnO) is a new type of high-function fine inorganic product with the particle size between 1 and 100 nm in the 21st century, exhibiting a variety of special properties such as non-migratory, fluorescence, piezoelectric, absorption and UV scattering ability, etc. With its special optical, electrical, magnetic and sensitivity performance, it can be used to produce gas sensors, phosphors, rheostat, UV shielding materials, image recording materials, piezoelectric materials, varistors, efficient catalysts, magnetic materials, and plastic films, etc.
Currently the methods producing zinc oxide mainly include chemical precipitation method, sol-gel method, microemulsion method and hydrothermal synthesis method, etc. But all raw materials used are zinc calcine or pure zinc salts (such as zinc sulfate, zinc nitrate or zinc acetate) with the zinc content higher than 50%.
Ammonia method is a commonly used method for producing zinc oxide. Currently, the ammonia method (ammonia-ammonium bicarbonate combined leaching method) for producing zinc oxide generally includes the following steps: leaching of zinc-containing materials using ammonia-ammonium bicarbonate as leaching agent, and after purification, ammonia evaporation crystallization, drying, calcinations of zinc-amine complexing solution, produce the zinc oxide product.
Above-mentioned traditional ammonia method for producing zinc oxide has not been used in electrolytic zinc acid leaching residue, mainly due to the following reasons:
1) Electrolytic zinc acid leaching residue is wrapped by a large amount of calcium sulfate, which cause it difficult to leach out zinc and the recovery rate is low.
2) Electrolytic zinc acid leaching residue contains 15-30% sulfate radical, so a large amount of ammonia converts into ammonium sulfate, with high ammonia consumption;
3) The zinc-ammine complexing solution after leaching is directly purified by conventional method, and finally the heavy metal is removed by zinc replacement method. Since there exists a large amount of free ammonia in the solution, with strong impurity complex, it is easy to produce redissolution phenomenon, affecting the purification effect and finally affecting the purity of zinc oxide product.
4) Due to the presence of large amount of ammonium sulfate in liquid, part of zinc ammonium sulfate double salt precipitation will occur in the ammonia evaporation process, and finally affecting the purity of zinc oxide.
Currently, the disclosed technologies of producing nanometer zinc oxide by ammonia leaching method are low-temperature hydrolysis methods, for example:
Chinese Patent Application No. 92103230.7 discloses an improved technology for producing zinc oxide aiming at traditional ammonia complexometry. The purified zinc-ammonia complexing solution is diluted with water to hydrolyze part of zinc-ammonia complexing solution and obtain the basic zinc carbonate (with a ratio of zinc hydroxide and zinc carbonate of 2:1), and then continue to heat until zinc-ammonia complexing solution is decomposed completely. After high-temperature calcinations, 30-100 nn nanometer zinc oxide is obtained.
For the patented technology, the following problems need to be solved:
After hydrolysis, in the thermal decomposition process of undissociated zinc-ammonia complexing solution, the newly generated basic zinc carbonate will continue to grow on the original surface of crystal nucleus, promoting the growing of originally hydrolyzed crystal, which is prone to cause uneven zinc carbonate crystal, making it difficult to control the particle size of the final product.
It adds 4-10 times of water, reducing the efficiency in the preparation process, increasing the energy consumption and the water treatment cost at the back end.
Chinese Patent Application No. 200610130477.7 disclosed an improved technology producing zinc oxide for the traditional ammonia complex method. The zinc-ammonia complexing solution is mixed with 1:2-20 of hot water or hot mother liquor continuously. After heating and heat preservation, it is recycled to be used in hydrolysis of zinc-ammonia complexing solution, to prepare 10-50 nn of nanometer zinc oxide.
For the patented technology, the following problems need to be solved:
After hydrolysis of mother liquor, the ammonia cannot be fully separated, and it cannot achieve the hydrolysis effect, and finally the zinc-ammonia complexing solution is mixed with the zinc-ammonia complexing solution.
For the above two patents, the nanometer crystals are obtained by slightly changing pH value with a large amount of water. In fact, relying solely on the pH value slight change, only a very small part of hydrolysis can be achieved (checked from the ammonium hydroxide solubility curve of zinc oxide). The higher concentration of zinc ammonia liquid, the higher the precipitation efficiency and lower energy consumption; while the lower concentration of zinc ammonia liquid, the lower the precipitation efficiency and high energy consumption. It is technically feasible by artificially increasing the proportion of water to produce nanometer zinc oxide, but it is not feasible in terms of economic efficiency.
In addition, for the current ammonia leaching method for producing zinc oxide, the crystal is basic zinc carbonate, with high decomposition temperature (the initial temperature of zinc hydroxide decomposition is about 125° C., and that of zinc carbonate is about 300° C.). In order to obtain high-purity products, it is necessary to guarantee a high decomposition temperature, generally controlled at above 500° C., to completely decompose the basic zinc carbonate. For example, in the Chinese Patent with Application No. 200610130477.7, the calcinations temperature is as high as 550° C. High-temperature calcinations seriously affect the specific surface area and dispersity of zinc oxide, and thereby affecting its application field.
In summary, for treatment process of the electrolytic zinc acid leaching residue, how to effectively leach the zinc from materials with low zinc content and get high-purity nanometer zinc oxide and to overcome the disadvantages of traditional pyrometallurgy and hydrometallurgy have become technical problems urgently to be resolved in the industry.